|Sigurd is one of the best choices for a cavalry tank thanks to his unique skills that can heavily mitigate damage taken from magical opponents. Most notably, despite the disadvantage, Sigurd can stand up to Reinhardt, who is normally the bane of any red unit.
Weapon: Divine Tyrfing is Sigurd's weapon of choice, and it is unique to him. It grants +3 Res, and more importantly, it mitigates damage taken from magic by 50%. This means that despite Sigurd's low Res, he effectively has a free Aegis against any mage, which improves Sigurd's survival greatly. This effect is also why a -Res bane is fine on him since he already has great damage reduction from mages anyway, and he has a good amount of HP to stomach the hit.
Assist: Reposition or Swap are Sigurd's choices of Assists. Reposition is more versatile, allowing the user to help an ally jump ahead and closer to their foes, or it can be used to help get allies out of danger. Swap performs a similar task but the user instead switches places with an ally, and is better used in tighter spaces. Either Assist will do the job, so just choose whichever is preferred.
Special: Bonfire or Ignis are Sigurd's choices for his Special. Both of them add damage based off of the user's Def, and Sigurd has plenty of that to take advantage of. This also comes down to player preference, with Bonfire having a faster charge and Ignis being far stronger.
A Passive: Distant Counter is chosen as Sigurd's A skill. With this skill, he can retaliate against any mage that dares to try and engage him. Most mages are pretty fragile and will be destroyed when Sigurd strikes back. He'll also be able to counterattack against most archers that try to fight him.
B Passive: Crusader's Ward is Sigurd's B skill, and is unique to him. If Sigurd is attacked twice consecutively from a range, then the damage of the second strike is reduced by 80%. This is the key skill that allows Sigurd to live against Reinhardt, but it also helps him survive against archers like Brave Lyn or other archers who wield the Brave Bow+. Between his weapon's effect and this skill's effect, Sigurd fears almost nothing from a range.
C Passive: Hone Cavalry is an option for Sigurd's C skill. Given that he is best used on a cavalry team, then a cavalry buff will help him support buff his teammates. However, other cavalry buffs can be used instead, depending on what is needed. Should he be used on a regular team, then regular Hone or Spur buffs could suffice, or even his his natural Spd Smoke 3 can be put to good use to help deter opponents by reducing their Spd.
Sacred Seal: Distant Def 3 or Quickened Pulse are both good options for Sigurd's Seal. Distant Def will help make Sigurd even sturdier against ranged opponents, thus improving his survival. Quickened Pulse on the other hand will help Sigurd charge his Special much faster, which can help him break through tougher opponents like Xander and Brave Ike.